The current transport of the mesoscopic SNS junction, as described in T. Matsui and H. Ohta “Low-Voltage Negative Resistance Mixers of Nano-Meter SNS Junctions” (IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 11, NO. 1, pp. 191-195, MARCH 2001) (hereinafter referred to as “Non-Patent Document 1”) or T. Matsui and H. Ohta. “Millimeter-and Submillimeter-Wave Negative Resistance SNS Mixers” (IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 9 NO. 2, pp. 191-195, JUNE 1999) (hereinafter referred to as “NonPatent Document 2”), is expressed by the resonance phenomenon of the multiple-Andreev reflections and the energy gap that occurs in the two SN interfaces. It is known that when the voltage is 0 V as shown in FIG. 1, namely in the voltage region infinitely approximating zero, the voltage-current characteristic of the mesoscopic SNS structure having an extremely small N region (or S′ region) as a junction area is transported by an infinite number of times (a very large number of times) of multiple-Andreev reflections and the superconducting electron-pair current assumes the largest amplitude and that when the voltage is in a finite state, the superconducting electron-pair current which quickly decreases in proportion as the voltage increases and the quasi-particle current which increases in accordance with voltage are synthesized and the negative resistance part reflecting the decrease of the superconducting electron-pair current occurs in the low voltage region. The low-noise amplifier that utilizes this negative resistance is disclosed in JP-A HEI 6-53762 (hereinafter referred to as “Patent Document 1”).
The transportation of current in the SNS element while the element is in the state of such low voltage as extremely approximating zero V is expressed by the formula (7) in Non-Patent Document 2. The amplitude of this electron-pair current reflects the multiple-Andreev reflections which quickly decrease in accordance as the bias voltage increases and, in the low voltage region having the bias voltage in the neighborhood of 0 V assumes a characteristic possessing negative resistance as shown in FIG. 1.
When an element possessing this characteristic is disposed in series connection, the superconducting electron-pair current that allows flow of the bias voltage at 0 V makes no change. The electron-pair current in the low-voltage region and the quasi-particle current flowing in the region exceeding 0 V, however, are shifted to a large voltage region depending on the number of SNS elements disposed in series connection. This is because the bias voltage is distributed by the series connection to the component SNS elements. As a result the voltage range of the negative resistance region part is expanded as shown in FIG. 2 and the dynamic range thereof is also expanded. The DC bias of the element that exhibits the negative resistance of this sort necessitates use of a voltage source (zero output resistance) therefor. By having the voltage bias set in the negative resistance region of the SNS element similarly to the structure of the Esaki diode known as a diode capable of exhibiting negative resistance it is rendered possible to extract the high-frequency current flowing in the element as a high-frequency current amplified by the large negative resistance. Then by having a biased SNS element disposed in the negative resistance voltage region in the resonant structure possessing a specific resonance frequency, it is rendered possible to realize a high-frequency oscillator capable of reaching even more than several hundred GHz, enable the negative resistance region dilated by serialization to exhibit a high output characteristic, and acquire a great advantage as a superconductor element restricted within a very low electric power region despite extremely low noise. Thus, in this invention, the negative resistance region of the SNS element begins at the position of breaking the state of zero voltage unlike the case of a tunnel diode wherein the negative resistance region is inevitably shifted toward high voltage depending on the number of series. Since the negative resistance region is spread out in a wide voltage region by the series connection, the condition for setting the bias in the neighborhood of the most sensitive zero voltage region becomes the voltage position in a practicable range. Though the receivers of millimeter wavelength to submillimeter wavelength bands assume grave necessity of satisfying the condition for matching the impedance of the whole element with the electromagnetic waves, they are at an advantage in easily accomplishing a high impedance condition due to series connection and are capable of unusually effectively enhancing the practical technical task as compared with a sole element.
In the case of the tunnel diode, as the characteristic manifested by an individual piece is indicated by a dotted line and the characteristic manifested by two pieces in series connection is indicated by a solid line in FIG. 4, the dynamic range of electric current in the part manifesting negative resistance does not change, whereas the negative resistance region widens in proportion to the number of series and is inevitably shifted as a whole toward high voltage.
Though series connection of the SNS element has been heretofore tried, as shown in FIG. 4 through FIG. 6 of H. Ohta et al. “SHORT WEAK LINKS FOR 115 GHz MIXERS” (IEEE TRANSACTIONS ON MAGNETICS, Vol. MAG-19 NO. 3, pp. 601-604 JUNE 1983) (hereinafter referred to as “Non-Patent Document 3”), the structure of Non-Patent Document 3 has never succeeded in exhibiting a negative resistance characteristic. The cause for this failure, as ascertained by the present inventors is that the finish of the narrow line of normal conduction serving to connect the first and second superconductors is destitute of processing controllability because the finish of the junction area of normal conduction of the mesoscopic small SNS junction structure exerts a strong influence on the property.
As described above, the SNS element unlike the conventional tunnel diode, is enabled by series connection to acquire improvement in the negative resistance property thereof. To date, the series connection has never been handled from this point of view and none of the conventional cases of implementing series connection has succeeded in acquiring a negative resistance property.
An element having two-dimensionally disposed superconductors mutually linked with weak links as disclosed in U.S. Pat. No. 5,109,164 (hereinafter referred to as “Patent Document 2”) has been known. The superconductors disclosed herein, however, have varying sizes and the links keeping the superconductors in weak link have directions of arbitrary selection. Moreover, since the plurality of superconductors linked in all these directions are so disposed as to constitute a two-dimensional structure the position that has chanced to incite a change of proper evades precise location. Further, the negative resistance in the low-voltage region that is accepted as being characteristic of the SNS junction has never beet realized with satisfactory reproducibility.